Water Uptake

Root hair cells

Structure

Functions and adaptations

• A root hair cell’s function is to absorb water via osmosis and mineral ions via active transport from the soil, anchoring the plant firmly.
• It has a large surface area. Each cell has a long, narrow extension that projects out into the soil spaces.
  • This significantly increases the surface area available for absorption, maxmizing the rate of water and mineral uptake.
• It also has a thin cell wall that provides a short diffusion distance for faster movement of water into the cell.

Pathway of water through the plant

• Once water enters the root hair cell, it travels down a water potential gradient across the plant tissues. The exact sequence of structures it passes through is:
  • **Water in soil ** $\rightarrow$ Root hair cells $\rightarrow$ Cells of cortex $\rightarrow$ Xylem $\rightarrow$ Mesophyll cells

Process

1. Root hair cells: Water enters from the soil by osmosis because the cytoplasm of the root hair cell has a lower water potential than the surrounding soil water.
2. Root cortex cells: Water moves from cell to cell across the root cortex via osmosis until it reaches the central vasuclar cylinder.
3. Xylem: Water enters the hollow xylem vessels, where it is pulled upwards throught he stem to the leaves by the transpiration stream.
4. Mesophyll cells: Once in the leaves, water leaves the xylem and moves into the surrounding mesophyll cells, where it is used for photosynthesis or evaporates into the air spaces before transpiring out through the stomata.

Investigation of the pathway of water (staining experiment)

• Apparatus needed:
  • a fresh stalk of celery with leaves
  • beaker
  • water
  • water-soluble stain

1. Place the base of the plant stem or celery stalk into the beaker filled with the colored dye.
   • This is to allow the plant to take up the stained water via the transpiration stream.
2. Leave the plant in a warm, well-lit, and well-ventilated area for a few hrs.
   • Warmth and light maximize the rate of transpiration, speeding up water movement.
3. Cut thin cross-sections (transverse cuts) of the steam at various heights using a scalpel.
   • Allows you to see exactly which internal structures have carried the colored water.

Observations

• In the stem, there are distinct colored dots arranged in a ring. They are the xylem vessels within the vascular bundles, demonstrating that water travels up the stem specifically through the xylem.
• In the leaves, the colored dye will eventually spread into the veins, highlighting the continuation of the vascular system from the stem into the leaves.